You don't need Humanism. I went Admin -> Religious in my game and as I said before had three non-event rebellions and used harsh treatment three to four times maximum. The best thing to do is just liberally increasing local autonomy in freshly conquered provinces, to off-set nationalism. Overall I've found the benefits of no rebellions far out-weight the added income. And I haven't been slow on expansion, either - although a lot of the early stuff was done by vassals.